Method for regenerating phosphoric pickling bath



Patented July 20, 1937 PATENT OFFICE METHOD FOR REGENERATING- PHOS-PHQRIC PICKLING BATH Jean Adrien Marie Bigeon, Colombes, France,assignor to La Compagnie Nationale de Matieres Colorantes etManufactures de Produits Chimiques du Nord Reunies, EtablisscmentsKuhlmann, Paris, France, a corporatio of France No Drawing.

3 Claims.

My invention has for its object the regeneration of the pickling bathsconstituted by phosphoric acid, generally .used for ferrous metals.

It is a well known fact that commercial phosphoric acid, used at theconcentration of 10 to 12% of- P205 and at the temperature of about 800., constitutes for all ferrous metals such as cast iron, soft steel,shaped and rolled steel and generally speaking cast iron and steel ofany description and obtained in any manner whatever, a pickling agentthe action of which is much more regular than any other mineral acidsuch as hydrochloric or sulphuric acid.

This fact can be explained in several manners: either the phosphoricacid, by reason of its natural viscosity, does not enter as deeply ashydrochloric or sulphuric acids into the pores and intercrystallineinterstices of the metals; or else.

the viscosity of the inert combinations of iron and phosphoric acidformed from the beginning of the pickling in the microscopic recesses ofthe metal opposes the subsequent penetration of free acid into theserecesses or again'the combinations of iron and phosphoric acid, notsaturated in iron, become bound throughtheir free acid valences to themetal wall, according to the theory of adsorption and form a protectingcoat opposing any subsequent attack.

.Whatever'may be the mechanism of the pickling action of phosphoric acidon iron, the surfaces treated by this acid are perfectly smooth whereasthe pickling by means of hydrochloric or sulphuric acids produces roughand dull surfaces.

This pickling by means of phosphoric acid in a first bath is generallyfollowed by an inimersion in a second bath of acid iron phosphate whichdeposits on the metal a film forming a yielding and resistant supportfor the paints or varnish to be applied subsequently. This processensures a longer life for the painting of iron'worksthan any otherprocess of pickling, sanding included. On the other hand it is a knownfact that such pickling leads to a very high consumption 01 phosphoricacid.

The phosphoric pickling bath, very active a the beginning of .itsaction, becomes soon sluggish, while still containing in the free statemore than half of the Pros employed' A very small amount of iron indissolution is suflicient for paralyzing the pickling action of thephosphoric acid, probably by reason of the rapid .andconsiderableincrease of the viscosity of the bath which increase ofviscosity is due to Application March 5, 1936, Serial In France July 16,1935 the presence of a compound of phosphoric acid and divalent iron.

The regeneration of the bath by means of successive addition of freshphosphoric acid is delusive because it soon becomes incapable ofpickling, for a specific weight of 18 to 20 Baum, when the contents offree P205 are below 8%.

It is then necessary to get prematurely rid of the pickling bath. Part(about A) of this first bath may be used after dilution to 23% of freeP205 for forming the second bath i. e. the bath of acid phosphate ofiron. The remainder of the first bath is thrown away. When the picklingis effected in this manner the efiiciency of the P205 employed isunderneath 50%.

Now I have found that it is possible to regenerate the pickling bathwhich has become too sluggish by freeing the phosphoric acid combinedwith divalent iron, by means of an addition of an acids produces ironsulphate or ferrous chloride,-

non-ionized in the bath, which frees the phosphoric acid heldincombination by the iron.

-It is found that the. quality of the bath after the addition ofsulphuric or hydrochloric acid is strictly the same as before saidaddition. The steel sheets pickled in a bath of phosphoric acidregenerated by sulphuric or hydrochloric acid have the same smooth andbrilliant appearance as those which have been pickled in a bath ofphosphoric acid containing no'addition of another mineral acid.

Certain easy precautions should be kept in mind when performingthisregeneration.

In the first place it is indispensable to. never leave any freesulphuric or hydrochloric acid in the regenerated phosphoric bath. Tothis end the amount of sulphuric or hydrochloric acid added at anymoment into the bath must always be proportional to the amount of ironexisting at the moment considered in the bath. Theoretically fortransforming into ferrous sulphate or .chloride the totality of the ironexisting in the phosphoric bath to be regenerated, there should be addedabout 100 parts in weight of sulphuric acid at 66 Baum or 220 parts inweight of hydrochloric acid at 20-22" B. for'56 of iron. Practically themaximum is 100 parts of sulphuric acid at 60 Baum or 200 parts ofhydrochloric acid at 20-2 2 Baum for 50 of iron.

There is, in fact, no interest in setting in liberty, at each additionof sulphuric or hydrochloric acid, all the phosphoric acid combined withthe iron; what is important is only the maintenance of a constantpickling power.

The addition of sulphuric acid is efiected under the form of sulphuricacid at 60 Baum because such an addition into a bath at 85 C. presentsno danger whereas acid at 66 Baum would cause projections of liquid.

Before proceeding-With an addition of sulphuric or hydrochloric acid, Imeasure the iron contents by means of potassium permanganate. Thismeasuring method is the best for the purpose as it has been noticed thatalmost all the iron dissolved in the bath is in the ferrous state.However when the measuring is performed in a bath which has beensubmitted to a beginning of regeneration through hydrochloric acid, itis nec-' essary to observe the usual known precautions required by thepresence of hydrochloric acid.

In order to provide a constant speed of pickling, which is anindispensable condition for regular working and leads to less expense inlabor and heating steam, it is preferable to not add all the sulphuricor hydrochloric acid at once when the phosphoric bath is nearlyexhausted, but to add it in several times, for instance in four times,when the 'iron contents of the bath reach 13, 26, 39 and 52 kgs. percubic meter.

I have noticed that after the fourth regeneration, the viscosity of thebath is much smaller than if I had simply reinforced the bath throughadditions of phosphoric acid. For this reason the bath may be used downto contents of about 4% of free P205.

In practice the regeneration through sulphuric or hydrochloric aciddoubles the efiiciency of use of the phosphoric acid employed as'well asthe duration of the pickling bath.

As the solubility of the ferrous sulphate and iron chloride in thephosphoric bath is comparatively small, it is preferable not to add inall more than 100 kgs. of sulphuric acid at 60 B. or 200 kgs. ofhydrochloric acid at 20-22 B. for 240 kgs. of phosphoric acid at 57 B.(50% of P205) employed. If this precaution were not observed I wouldrisk producing in the phosphoric bath a crystallization of iron sulphateor chloride. As a. bath at 10-11% of P205 contains about 240 kgs. ofphosphoric acid at 50% of P205 per cubic meter, it is easy to bear inmind that the amount of sulphuric acid at 60 B. to add into it shouldnotrise beyond 100 kgs. per cubic meter of bath or that the amount ofhydrochloric acid at 20-22 B. should not rise beyond 200 kgs. per cubicmeter of bath.

It should be noted that the phosphoric pickling bath regenerated bymeans of, sulphuric or hydrochloric acid should never serve forpreparing the second bath or bath of iron phosphate.

For preparing this second bath I use solely an exhausted, but notregenerated phosphoric bath so as to introduce into the second orphosphate bath only these traces of iron sulphate or chlo-v rideintroduced by the iron sheets coming out of the phosphoric bath whichtraces the most sensitive reactions are incapable of showing and theeffect of which on the second or phosphate-bath can be but negligible.

Example 1 (sulphuric acid) Using a first or phosphoric pickling bath of10 cubic meters, containing originally 11% of P205 and kept at 0., theduration of pickling of a sheet iron which was at the beginnin 2 minuteshad risen to 30 minutes after 8 days of working.

The amount of divalent iron measured by means of potassium permanganateis then 14 kgs. per cubic meter of bath; the acidimetric measure of P205after elimination of the iron salts through the classical acetoneprocess gives out contents of 7%. I

If the second bath or iron phosphate bath requires being renewed at thismoment (this bath serves for the'duration of several pickling baths) Ithrow away this second bath and replace it by 2,500 litres of the firstor phosphoric bath and 7,500 litres of water. These figures correspondto a second bath of same volume as the first or phosphoric bath which isnot compulsory.

On the other hand the phosphoric first bath is returned substantially toits original volume by an addition of about 600 kgs. of phosphoric acidcontaining 50% of P205 (corresponding to the 240 kgs. contained in eachcubic meter of the phosphoric bath removed for reconstituting thesecond) and of the required volume of water.

I add then 250 kgs. of sulphuric acid at 60 B. which raise the contentsof free P205 to 9.5% and reduce the duration of pickling of a sheet ironto 15 minutes.

The pickling being then continued at the same rate analysis showssuccessively on the 10th, 12th and 14th days contents of iron of 26, 38and 51 kgs. per cubic meter. On each of these days I add 250 kgs. ofsulphuric acid at 60 B, i. e. in all 1000 kgs. After the fourthregeneration, the work may be continued up to the 15th day.

Example 2 (hydrochloric acid) A pickling bath of 10' cubic meterscontaining originally 10.5% of P205 and'held at 50 C. is found to be toosluggish after an eight. days use. The measuring of the divalent iron bythe permanganate method gives out contents of 12 kgs. per cubic meterwith 7.5% of free P205.

If the second or .phosphatation bath requires same rate and I find onthe 10th, 12th and 14th days, 25, 37 and 49 kgs. of ferrous iron percubic meter. I regenerate each time through an addition of 500 kgs. ofhydrochloric acid at 20-22 B. and I continue the pickling up to the15thday.

What I claim is:

1. In the method for pickling ferrous metals with phosphoric acid bathscontaining at least 10% P205 the step which consists in adding to thebath when the bath has lost part of its activity by combination of thephosphoric acid with iron, sulphuric acid in amounts not above thatcorresponding to saturation of the iron present in the bath.

2. In the method for pickling ferrous metals with phosphoric acid bathscontaining at least 10% P205, the step which consists in adding to thebath, when the bath has lost part of its activity by combination of thephosphoric acid with iron, parts in weight at the most of sulphuric acidat 60.Baum for 50 parts of iron present in iron, sulphuric acid inamounts not above that corresponding to saturation of iron present inthe bath and not exceeding 100 kgs. of sulphuric-acid at 60 Baum foreach 240 kgs. of phosphoric acid at 50% of P205 contained in the bath.

JEAN ADRIEN MARIE BIGEON.

